In an internal combustion engine, a catalyst for purging exhaust gas is arranged in an exhaust passage to purge exhaust components. The purging of exhaust components with the catalyst is efficiently performed when the air-fuel ratio of the air-fuel mixture burned in the internal combustion engine is within a predetermined range. To adjust the air-fuel ratio to be within the predetermined range, air-fuel ratio feedback control is normally executed to increase or decrease and thereby correct the injected amount of fuel supplied to the internal combustion. Specifically, an oxygen sensor, which detects the oxygen concentration in the exhaust gas, is arranged at the upstream side of the catalyst in the exhaust passage. In air-fuel ratio feedback control, the air-fuel ratio of the air-fuel mixture is determined based on an output signal of the sensor. To adjust the detected air-fuel ratio to a target air-fuel ratio, an air-fuel ratio correction value is obtained for the fuel injection amount. The fuel injection amount is increased or decreased and thereby corrected based on the air-fuel ratio correction value.
Further, a so-called air-fuel ratio sub-feedback control may be executed to calculate a revised value for the air-fuel ratio correction value. Specifically, an oxygen sensor, which detects the oxygen concentration in the exhaust gas, is arranged at the downstream side of the catalyst in the exhaust passage to determine the state of exhaust components purged by the catalyst. In the sub-feedback control, the revised value for the air-fuel ratio correction value is determined based on an output signal of the sensor.
When deterioration of the catalyst advances, the purging of the exhaust becomes insufficient even if the air-fuel ratio of the air-fuel mixture is properly controlled. Therefore, various types of devices for determining deterioration of the catalyst have been proposed in the prior art.
For example, in the device described in patent document 1, catalyst deterioration is determined in the following manner.
The catalyst occludes oxygen from the exhaust gas if the oxygen concentration of the exhaust gas passing through the catalyst is higher than the oxygen concentration of when the air-fuel mixture is burned at the stoichometric air-fuel ratio. If the oxygen concentration of the exhaust gas passing through the catalyst is lower than the oxygen concentration of when the air-fuel mixture is burned at the stoichometric air-fuel ratio, the catalyst releases the occluded oxygen. In this manner, the catalyst has an oxygen storage function. Thus, when changing the air-fuel ratio of the air-fuel mixture in the combustion chamber from a rich (low) state to a lean (high) state, the oxygen concentration of the exhaust gas at the downstream side of the catalyst becomes high after the catalyst completes oxygen occlusion. When changing the air-fuel ratio of the air-fuel mixture in the combustion chamber from a lean state to a rich state, the oxygen concentration of the exhaust at the downstream side of the catalyst becomes low after the catalyst releases oxygen. Accordingly, the oxygen occlusion amount of the catalyst may be estimated by monitoring changes in the oxygen concentration at the downstream side of the catalyst after changing the air-fuel ratio of the air-fuel mixture. The oxygen occlusion amount is apt to decrease as deterioration of the catalyst advances. Thus, in the device described in the above document, the oxygen occlusion amount of the catalyst for the above-described situation is estimated, and deterioration of the catalyst is determined based on the estimated value.
In an internal combustion engine, the fuel injection amount may undergo an increasing correction so that the air-fuel ratio of the air-fuel mixture becomes richer than the stoichometric air-fuel ratio such as during acceleration or the like. The amount of fuel relative to the air becomes excessive when the increasing correction of the fuel is performed. Thus, unburned fuel is apt to be discharged into the exhaust passage. Unburned fuel discharged in such a manner remains in the exhaust passage until a certain time elapses from when the increasing correction of the fuel injection amount ends. Therefore, when calculating the oxygen occlusion amount after the increasing correction ends, such residual unburned fuel may result in erroneous correction of the oxygen occlusion amount. This may adversely affect the deterioration determination of the catalyst.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-329832